This invention relates to a photographic image-processing method for subjecting an image to photographic enhancement in regard to the density information thereof.
Generally, it is necessary to enhance the density of an image having a continuous tone. In order to enhance the density a contour image consisting of an edge of a photographic image, i.e., a portion having a great difference in density is formed, or a contour image containing a few residual density portions therein is formed. The formation of such an image is utilized not only in the field of art, but also in the field of measuring concerning geological sectional maps, and distribution maps of plants, etc. This type of image processing can be performed electronically by using a photoelectric conversion device such as a flying-spot tube or the like. The electronic processing, however, has a drawback in that it requires expensive and complicated facilities. For this reason, a photographic process using a chemical treatment is generally employed. This photographic image-processing method includes, for example, a Tone Line Process described in Kodak Pamphlet No. Q-18, 66/10 published by Eastman Kodak Company. This process includes the following steps. (1) A positive film is made from a negative film having a continuous tone image. (2) The negative film is placed on the positive film in such a manner that their bases are facing each other and that their images are wholly in registration. In this step, the photosensitive emulsion layers of the films may also be arranged to face each other instead of the bases of the film. In the latter, a transparent film is sandwiched between the films. (3) The combined film unit obtained in step (2) is placed on a photosensitive sheet, such as a photographic film or paper, and the photosensitive sheet is exposed to oblique light by way of the film unit. The photosensitive sheet and light source are rotated relatively to each other to permit light to be obliquely irradiated onto the photosensitive sheet from all directions therearound. Since the images on the negative and positive films are wholly in registration, the combined film unit has a uniform density. For this reason, if the photosensitive sheet is exposed to light at a right angle, no image is formed on the photosensitive sheet. However, by exposing it to oblique light, a linear contour image is formed on the photosensitive sheet by way of the film unit. The relative rotation of the light source and photosensitive sheet eliminates the inconvenience of the oblique light being irradiated from only one direction which makes a contour of only one side of the image.
The above-mentioned prior art photographic image-processing method causes a variation in the linear image width unless the rotation is made on a uniform basis. However, the uniform rotation is difficult. Further, the linear image width is related to the angle of incidence of light with respect to the film unit and also to the thickness of the film or transparent film. This makes the setting of the linear image width difficult. Further, the prior art method enables only a linear image to be obtained, presenting difficulties in forming a contour image containing a few residual density portions therein.
There is another photographic image-processing method which is so called an "out-of-focus mask method". In this method, an out-of-focus positive film is formed by exposing a unit of positive film, negative film and transparent film sandwiched therebetween for scattering light. The positive film thus formed is placed on the negative film in registration to make a mask. A photosensitive sheet is exposed to light at a right angle by way of the mask. The contour image formed on the photosensitive sheet depends on the degree the mask is out of focus, so that the degree of contour enhancement is not anticipated unless the mask is formed. It is also difficult to form a mask, precisely.